This invention pertains generally to DS-CDMA cellular radio systems, and more particularly to a Synchronization Channel for asynchronous DS-CDMA base stations that facilitates cell search by a mobile station.
In conventional DS-CDMA (direct sequence code-division multiple access) systems, the downlink channels for each cell site are constructed by spreading the data with a long code. (The term xe2x80x9clong codexe2x80x9d connotes that the spreading code is much longer than data symbol periods.) A reason for spreading in such manner is so that when correlating with a particular long code so as to detect a downlink signal from a particular cell, downlink signals from other cells appear as noise and can thus be discriminated against. A mobile station (MS) finds the strongest cell site and acquires that cell site""s long code at the beginning of communication.
In synchronous cellular systems such as second generation IS-95, different time-shifted versions of a single code sequence are assigned to different cell cites, which enables cell search to be accomplished in a relatively short time. (The sequence is known, and only the time shift is to be determined for each cell site.) With this method, an external timing source is required to time-synchronize all cell sites to within a few microseconds. A commonly used external timing source is the Global Positioning System (GPS), with GPS receivers deployed at the cell sites.
However, some cell sites are unable to receive GPS signals, particularly those that are located indoors. In order to flexibly deploy the system in which mobile stations may transmit from outdoors to indoors, it might be advantageous to use asynchronous cell site operation because it does not require the external timing source. In asynchronous cell site operation, however, a different long code is assigned to each cell in order to distinguish one cell from another. Thus, several hundred long codes might be required throughout the system. In general, the cell search time in asynchronous systems is much longer than in synchronous systems since all the long codes used in the system are searched.
In third generation CDMA systems such as the UMTS system which operate in asynchronous mode, a special Synchronization Channel (SCH) is used in the downlink to speed up acquisition of a cell site. If the number of scrambling codes used in the system is L=MS, then these codes might be subdivided into M code groups each consisting of S scrambling codes. A three-step cell search algorithm has been chosen for the initial UMTS cell site acquisition using this concept of code groups: i) the mobile station detects a short spreading code common to all cell sites to acquire initial slot timing; ii) the mobile station then determines which of the M code groups the base station belongs to, and; iii) the final step is to search the S scrambling codes within the code group to acquire the actual long scrambling code of the base station.
The Synchronization Channel (SCH) carries a downlink signal used by the mobile stations for cell search in the first and second step of the initial cell site acquisition. The SCH consists of two sub-channels, the Primary SCH and Secondary SCH.
The Primary SCH carries an unmodulated code of length 256 chips, the Primary Synchronization Code (PSC), that is transmitted once every slot. The PSC is a sequence common to every cell in the system.
The Secondary SCH repeatedly transmits a sequence of unmodulated codes each of length 256 chips, the Secondary Synchronization Codes (SSC). These codes are transmitted in parallel with the codes transmitted on the Primary SCH. Conventionally, the SSC is structured to form a comma-free code word within the consecutive slots of one frame. The symbols in the code word are taken from a fixed set of codes of length 256. The order of symbols and which symbols to use are conventionally determined by a group-specific code word, for example a comma-free subcode of a Reed-Solomon code.
Each comma-free code word represents a scrambling code group identity, and since all code words are distinct under symbol-wise cyclic shifts, the frame timing can also be determined. In particular, M sequences are used to encode the M different code groups each containing S scrambling codes. The M sequences are constructed such that cyclic shifts of their constituent bits are unique, i.e., a non-zero cyclic shift of the bits of any one of the M sequences is not equivalent to some cyclic shift of any of the remaining Mxe2x88x921 sequences. Also, a non-zero cyclic shift of any one of the sequences is not equivalent to any other cyclic shift of itself. This property is used to uniquely determine both the long code group and the frame timing in the second step of cell acquisition.
In one realization of the UMTS Synchronization Channel, each of the SSCs is constructed by the position-wise addition modulo 2 of a Hadamard sequence (different for each SSC) and a hierarchical sequence used also on the Primary SCH. Cross-correlation values between the conventional PSC codes and SSC codes are not as low as desired. In some cases, the aperiodic cross correlations values between PSC and SSC can be up to 70% of the main peak of the auto-correlation function. There is thus a need for codes of limited length with better correlation (auto- and cross-correlation) properties. There is also a need for a code with a structure which can be correlated for detection with low complexity and in a short time interval to improve the acquisition performance of the secondary SCH.
In accordance with the teachings of the present invention, these and other advantages may be accomplished by the present systems and methods of providing an improved synchronization channel in CDMA systems. An embodiment of the present invention includes a method for a mobile station to identify a base station and to synchronize with frame timing of the identified base station, comprising: selecting in each base station a different first number from a first predetermined set of M numbers each of a first length N2; selecting in each base station a second number associated with the first number from a second predetermined set of M numbers each of a second length N1 where N1 is greater than or equal to the number of slots in a frame; transmitting from each base station a synchronization code of length N=N1xc3x97N2 digits determined in each slot by: generating each digit position of the synchronization code by: selecting a first digit from the first number according to a function of digit position in the synchronization code; selecting a second digit from the second number according to a function of digit position in the synchronization code; and adding the first and second digits in modulo arithmetic, and cyclically shifting the second number after each slot, and correlating in a mobile station samples of synchronization code received from a base station starting simultaneously with a start of a slot with combinations of: M values of the first number, and all cyclic shifts of each corresponding second number to produce Mxc3x97N1 decision variables; and identifying a maximum decision variable, wherein a particular first number corresponding to the maximum decision variable identifies the first number selected at the base station, whereby the base station is identified; and wherein a particular cyclic shift corresponding to the maximum decision variable identifies the slot whose start corresponds to starting of a frame.
Another embodiment of the invention provides a propagated signal divided into frames, each frame comprising two or more slots one of which starts simultaneously with frame start and comprising a synchronization code determined by: selecting a first number from a first predetermined set of M numbers each of a first length N2; selecting a second number associated with the first number from a second predetermined set of M numbers each of a second length N1 where N1 is greater than or equal to the number of slots in a frame; determining a synchronization code of length N=N1xc3x97N2 digits in each slot by: generating each digit position of the synchronization code by: selecting a first digit from the first number according to a function of digit position in the synchronization code; selecting a second digit from the second number according to a function of digit position in the synchronization code; and adding the first and second digits in modulo arithmetic, and cyclically shifting the second number after each slot.
The invention will next be described in connection with certain exemplary embodiments; however, it should be clear to those skilled in the art that various modifications, additions and subtractions can be made without departing from the spirit or scope of the claims.